In a process of producing a general bipolar integrated circuit in which semiconductor elements are combined, the production process becomes complicated if a n-p-n type transistor and a p-n-p type transistor are arranged in the same chip in such a manner that an emitter junction is opposed to a collector junction. Accordingly, a lateral transistor operating in a lateral direction and having a structure shown in FIG. 2 is used as a p-n-p type transistor in the integrated circuit.
In FIG. 2, numeral 1 denotes a p-type semiconductor substrate, numeral 2 denotes a n.sup.+ type buried layer, and numeral 3 denotes a n-type epitaxial layer which is a first conductivity-type layer formed on the semiconductor substrate 1. Numeral 4 denotes a n.sup.+ -type layer for connecting a base electrode wherein impurity such as a phosphorus is diffused with high concentration in the n-type epitaxial layer 3. Numerals 5 and 6 denote p.sup.+ -type layers, which are second conductivity-type layers, wherein acceptor impurity such as a boron and, a boron fluoride is diffused to form a collector zone and an emitter zone respectively. The collector zone 5 and the emitter zone 6 are generally formed just when a p-type diffusion of another n-p-n transistor (not shown) is formed.
With respect to the lateral p-n-p type transistor stated above, current flows in the neighborhood of a surface of the transistor due to its structure. Accordingly, the is p-n-p type transistor easily effected by the surface thereof and current amplification factor h.sub.fe is generally small. The current amplification factor h.sub.fe can be increased by allowing a positive hole from the emitter to effectively reach the collector by narrowing a base width, i.e. a distance (W.sub.b in FIG. 2 ) between the collector zone 5 and the emitter zone 6 or enlarging the collector.
In order to narrow the base width in this kind of transistor, the spacing between the collector zone and the emitter zone must be narrowed when the collector zone 5 and the emitter zone 6 are formed by diffusion of impurity. Forming accuracy cannot be, however, improved so much due to limitations in accuracy of a photomask, adjusting accuracy in transfer, pattern definition of a resist, exposure and etching accuracy, occurrence of a defect caused by dust, and the like. Thus, there are limitations in narrowing the base width.
Further in the conventional structure, the collector zone 5 has the same depth as the emitter zone 6 since the collector zone 5 is formed just when the emitter zone 6 is formed, that is, the collector zone 5 and the emitter zone 6 are simultaneously formed. Thus, many positive holes injected through the emitter move under the collector zone 5 and directly reach the base electrode 4, or directly reach the semiconductor substrate 1 via a buried layer 2. Therefore, there is a problem that the positive holes which do not reach the collector zone are increased so that the current amplification factor cannot be increased.